Splitting of water molecules driven by ultrasound plays a central role in sonochemistry. While studies of sonoluminescence revealed the formation of a plasma inside the cavitation bubble, much less is known about the contribution of plasma chemical processes to the sonochemical mechanisms. Herein, we report for the first time sonochemical processes in water saturated with pure CO. The presence of CO causes a large increase in the H/D kinetic isotope effect (KIE) to $\alpha_H$ = 14.6 ± 1.8 in a 10% H$_2$O/D$_2$O mixture under 20 kHz ultrasound.The anomalous H/D KIE is attributed to electron quantum tunneling in the plasma produced by cavitation. In addition, CO$_2$ formed simultaneously with hydrogen during the sonochemical process is enriched with the $^{13C}$C isotope, which indicates a VV pumping mechanism typical for non-equilibrium plasma. Both observed KIEs unambiguously point to the contribution of quantum effects in sonochemical mechanisms.